Field of the Invention
This invention relates generally to correction waveform generation in an image display, and more particularly to a waveform generator operable at a plurality of display standards.
Waveforms comprising multiple frequency components frequently include a DC component which renders the signal essentially unipolar. However, removal of the DC component, for example, by capacitive coupling results in the loss of the unipolar characteristic with the resultant waveform being disposed equally in area, positively and negatively about an average waveform value. This average value is dependent on the waveform shape, thus when AC coupled waveforms with differing shapes produce differing average values with respect to the waveform peaks. Hence the AC peak potentials received by the circuitry following the AC coupling are altered, and vary in accordance with differing waveform shapes.
In an exemplary cathode ray tube the distance from the center of electron beam deflection is, in general, shortest to the center of the display screen, with the distance increasing to maximum values in the screen corners. Thus to achieve consistent beam landing or a focused electron beam over the complete screen area requires that a DC focus voltage is combined with a signal waveform comprising multiple frequencies, for example, horizontal and/or vertical frequency parabolic shaped waveforms. Typically this parabolic waveform is generated with low voltages near the system ground potential and added to the high voltage DC focus voltage via an AC coupling. The amplitude of this parabolic signal has a factory determined value, since the distances between all screen locations and the center of electron beam deflection are known and fixed. Thus, a single focus control, which adjusts a DC potential may be provided to allow optimum focus to be obtained, not only in the screen center, but at all screen locations. Such overall optimized adjustment assumes an accurately determined factory set amplitude value for the generally parabolic shaped signals.
Although the geometric relationship between the display screen and electron beam are fixed and hence not standards specific, a display device may be capable of operation at multiple display standards with various scanning frequencies and differing retrace and blanking times. Thus a parabolic waveform generator is required which is responsive to the display standard, follows the scanning frequency, is capable of differing phasing relative to a vertical retrace pulse and is responsive to differing blanking durations. Such varieties of waveform shaping and phasing consequently vary the AC peaks with respect to the DC component of the waveform. Hence, when this exemplary waveform is ultimately AC coupled for addition to the high DC voltage for DC focus control, the loss of the DC component of the waveform may necessitate readjustment or optimization of the DC focus control voltage. Hence a display operable at multiple scanning and display standards, may require individual focus control adjustment for each display standard.
U.S. Pat. No. 5,471,121 discloses a circuit for generating a dynamic focus voltage. The focus voltage is compared to a reference voltage. Whenever the focus voltage drops below the reference voltage, a switching signal is provided. The switching signal controls a switch, which provides a DC focus voltage during the overscan portions of the trace period. The DC focus voltage has such a value to keep the average value of the focus voltage essentially constant.
In EP-A 0 554 836 a parabolic waveform generating apparatus is described. The known apparatus comprises two memory devices, each storing data necessary for generating parabolic waveforms necessary for the images of a specific aspect ratio. For different aspect ratios an arithmetic logic unit processes the output data of the two memory devices to generate new output data values for a desired parabolic waveform.
DE-A 197 54 904 provides a method for generating a focus voltage for a CRT. The known method comprises the steps of verifying if reference data have changed, calculating correction data and generating a focus voltage according to the correction data.
It is an object of the preset invention to provide a method for generating display correction waveforms for a CRT display. This object is solved by a method according to claim 1. The inventive method comprises the steps of selecting one of a plurality of trace portions for forming part of a correction waveform, the trace portions having different average values. Completing each of the correction waveform by combining each selected trace portion with a respective retrace portion such that all completed correction waveforms have a predetermined average value. Further advantageous modifications of the inventive method are subject of dependent subclaims.
It is another object of the present invention to provide an apparatus having improved focus voltage generation. This object is solved by an apparatus according to claim 1.
It is an advantage of the apparatus according to the invention that independent of the actual waveform modulating the focus voltage a predetermined average value is maintained. In this way a well focused electron beam is yielded over the whole screen area.
An embodiment of the inventive apparatus can be equipped with means for superimposing waveforms for vertical and horizontal focus voltage modulation. It is noted that the maintenance of a predetermined average value of the modulating waveform is achieved for vertical and horizontal focus voltage modulation.
Further advantageous modifications of the inventive apparatus are subject of dependent subclaims.